With a house that old, you could easily uncover issues that cost 100k to repair, like failing structure, foundation, electrical (knob and tube was the standard when the house was originally wired -- this place is so old it predates electricity), heating/cooling (old boiler, furnace, coal chute even?), and you already mentioned the septic.
That's not to say you should avoid it. Just do extra due diligence before you buy. Double your expected repair expenses (5% is too low to begin with), and find a home inspector who's highly recommended and knows what to look for in old houses. Also, go check and see what permits have been pulled on the property over the years. If the electrical/hvac/plumbing was upgraded, that work should've been permitted, which means you can see when it was done and who did the work.
If the numbers you posted are accurate and a home inspector finds no major issues, it looks like a pretty good deal.